1. Field of the Invention
The present invention relates to voltage control oscillation circuits, and more particularly to improvements in voltage control oscillation circuits for use in signal processing circuits of audio equipment, imaging equipment or the like.
2. Prior Art
FIG. 1 shows a conventional voltage control oscillation circuit. In FIG. 4, numeral reference 31 denotes a charge-discharge switching circuit, 32 a capacitor, 33 a discharge resistor, and 34 a control voltage input terminal.
The charge-discharge switching circuit 31 is constituted by a circuit for charging and discharging the capacitor 32 alternately, and charging voltage has two threshold values, that is, an upper and a lower threshold value. When a charging voltage reaches the upper threshold value in the capacitor 32, the charging of the capacitor 32 is suspended. Then, the capacitor 32 is discharged by the discharge resistor 33 which is connected thereto in parallel, as a result of which the charging voltage drops. When the charging voltage drops up to the lower threshold value, the charge-discharge switching circuit 31 operates so as to charge the capacitor 32.
Since the capacitor 32 is charged with current surpassing discharging current by means of the discharge resistor 33, the charging voltage rises and reaches the upper threshold value in the capacitor 32. The process mentioned above is repeated in the same manner so that a saw tooth or triangular oscillating signal is output.
Further, either upper or lower threshold value varies according to the control voltage received via the control voltage input terminal 34, thus causing the amplitude of the oscillating signal to vary. On the other hand, charge-discharge constants for use in defining the inclination of the oscillating signal and the like are fixed. Therefore, the oscillating frequency varies with the repetitive period. The oscillating frequency of the output signal is thus controlled by the control voltage.
However, an oscillating circuit of the sort set forth above needs an externally-installed capacitor with relatively large capacity, and this tends to render such an oscillating circuit liable to frequency fluctuations due to aging change of the capacitor. FIG. 2 illustrates an exemplary voltage control oscillation circuit using a ceramic or quartz oscillator 41 in order to improve aging and temperature change characteristics. In this case, the oscillating frequency of the output signal is still controlled as charge-discharge time constants to capacitors 42 and 43 vary with the control voltage via a terminal 44.
FIG. 3 illustrates an exemplary oscillation circuit using an operational amplifier, wherein oscillation is carried out by charging and discharging a capacitor 52 via a resistor 51, and a capacitor 54 via a resistor 53. The resistance of this circuit is replaced with a circuit equivalent to a variable resistor, though the illustration thereof has been omitted. Moreover, the oscillating frequency is controlled as the apparent resistance value of the equivalent circuit varies with the control voltage.
The circuit shown in FIG. 1 requires an external regulating circuit to secure an accurate oscillating frequency. In the case of such a conventional voltage control oscillation circuit, it is impossible to make the whole circuit an IC even though an attempt is made to do so. Moreover, a capacitor of large capacity is required for the charge-discharge switching circuit 31 shown in FIG. 1 and the operational amplifier circuit shown in FIG. 3 to obtain an oscillating signal at a stable oscillating frequency. In other words, such a capacitor must be connected to the circuit as an external part because it is hardly possible to incorporate the capacitor into an IC. Consequently, the oscillating frequency tends to fluctuate because of the aging change of the capacitor.
Particularly when a low-frequency oscillating signal is needed, the oscillating condition will become unstable under the influence of leakage current, noise or the like if the charge-discharge time constants are increased simply by increasing the resistance value of a built-in resistor. As a result, an extremely large capacity capacitor will have to be used instead. However, such a large capacity capacitor has disadvantages, in addition to those mentioned above, in that it is not only expensive but also inferior in reliability. Moreover, there arises such a problem that it may incur a reduction in packaging density.
The practice usually taken up to remedy the aforesaid disadvantages is to use, for example, a ceramic oscillator for generating a high-frequency oscillating signal once (FIG. 1) and then to divide the frequency. However, the necessity of dividing the frequency repeatedly until the frequency required is obtained would result in a sharp increase in the area of the frequency divider on chip. A resulting increase in the chip area tends to decrease a yield of chips per wafer and the yield itself, thus increasing IC production cost. In view of securing desired packaging density, the arrangement stated above is disadvantageous because the oscillator has to be installed externally.